These days, it is predominantly microlithographic projection exposure methods that are used for producing semiconductor components and other finely structured components, such as e.g. photolithography masks. Here, use is made of masks (reticules) or other pattern generating devices, which carry or form the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in the region of the object plane of the projection lens between an illumination system and a projection lens in a projection exposure apparatus and it is illuminated by illumination radiation provided by the illumination system. The radiation modified by the pattern travels through the projection lens as projection radiation, the projection lens imaging the pattern with a reduced scale onto the substrate to be exposed. The surface of the substrate arranged in the image plane of the projection lens optically conjugate to the object plane. The substrate is generally coated with a radiation-sensitive layer (resist, photoresist).
One of the goals in the development of projection exposure apparatuses involves producing structures with increasingly smaller dimensions on the substrate by way of lithography. In the case of e.g. semiconductor components, smaller structures lead to higher integration densities; this generally has an expedient effect on the performance of the microstructured components produced.
The size of the structures that can be produced depends crucially on the resolving power of the employed projection lens and the latter can be increased, firstly, by reducing the wavelength of the projection radiation used for the projection and, secondly, by increasing the image-side numerical aperture NA of the projection lens used in the process. These days, projection exposure apparatuses including high-resolution projection lenses operate at wavelengths of less than 260 nm in the deep ultraviolet (DUV) range or in the extreme ultraviolet (EUV) range.
Projection lenses generally have a multiplicity of optical elements in order to meet partly conflictting desired properties with regard to the correction of imaging aberrations possibly even with large numerical apertures used. Both refractive and catadioptric projection lenses in the field of microlithography often have ten or more transparent optical elements. In systems for EUV lithography it is endeavoured to manage with the fewest possible reflective elements, e.g. with four or six mirrors.
Besides the intrinsic imaging aberrations that a projection lens may have on account of its optical design and production, imaging aberrations may also occur during the use period, in particular during the operation of a projection exposure apparatus on the part of the user. Such imaging aberrations are often caused by changes in the optical elements installed in the projection lens as a result of the projection radiation employed during use. This is often dealt with under the key words “lens heating”. Other internal or external disturbances can also lead to the impairment of the imaging performance. They include, inter alia, a possible scale error of the mask, changes in the air pressure in the surroundings, differences in the strength of the gravitational field between the location of the original lens adjustment and the location of use by the customer, changes in refractive index and/or shape alterations of optical elements on account of material alterations as a result of high-energy radiation (e.g. compaction), deformations on account of relaxation processes in the holding devices, drifting of optical elements and the like.
Modern projection exposure apparatuses for microlithography include an operating control system, which allows a near-instantaneous fine optimization of imaging-relevant properties of the projection exposure apparatus to be performed in reaction to environmental influences and other disturbances. For this purpose, at least one manipulator is actuated in a manner appropriate to the current system state in order to counteract a disadvantageous effect of a disturbance on the imaging performance. In this case, the system state can be estimated e.g. on the basis of measurements, from simulations and/or on the basis of calibration results or can be determined in some other way.
The operating control system includes a subsystem—belonging to the projection lens—in the form of a wavefront manipulation system for dynamically influencing the wavefront of the projection radiation travelling from the object plane to the image plane of the projection lens. In the course of dynamic influencing, the effect of the components of the wavefront manipulation system arranged in the projection beam path can be adjusted in a variable manner depending on control signals of the operating control system, as a result of which the wavefront of the projection radiation can be modified in a targeted manner.
The optical effect of the wavefront manipulation system can be modified in the case of e.g. specific, predefined occasions or in a manner dependent on the situation prior to an exposure, or else during an exposure.
The wavefront manipulation system includes at least one manipulator having at least one manipulator surface arranged in the projection beam path. In this case, the term “manipulator” denotes devices configured, on the basis of corresponding control signals of the operating control system of the projection exposure apparatus, to actively influence individual optical elements or groups of optical elements in order to change the optical effect thereof, in particular change it in such a way that an aberration that occurs is at least partly compensated for.
A manipulator contains one or more actuating members or actuators, the current manipulated value of which can be changed or adjusted on the basis of control signals of the operating control system as a result of a manipulated value change. A manipulated value change can bring about e.g. a displacement or deformation of an optical element. If a manipulated value change is a movement of an actuator, e.g. in order to displace or tilt an optical element, then a manipulated value change can also be referred to as “manipulator travel”. A manipulated value change can also be present e.g. as a temperature change or as a change in an electrical voltage.
A manipulated value change brings about a change in the imaging properties that can be influenced by the manipulator (at least one). The efficacy of a manipulator vis-à-vis specific imaging aberrations is usually described by the so-called “sensitivity” of the manipulator to the imaging aberrations. The term sensitivity describes the relationship between a defined manipulated value change at a manipulator and the effect achieved thereby on the imaging quality or on lithographic aberrations.
In known operating control systems, the manipulated value changes at manipulators, or at actuators of manipulators, which are involved for a desired intervention in the system are determined on the basis of a control programme with a correction algorithm which optimizes a target function (merit function). What is thus intended to be achieved, inter alia, that, rather than an individual residual aberration being minimized at the cost of others, an expedient, balanced reduction of all relevant influencing variables occurs.
The European patent EP 1 251 402 B1 describes an operating control system which uses a target function. In this case, the target function describes the quality of the exposure process as a weighted sum of a multiplicity of “lithographic aberrations”. In this case, the term “lithographic aberration” is intended to encompass all defects relevant to lithography during the imaging. The lithographic aberrations include, inter alia, aberrations such as distortion (non-uniform displacement of image points in the image field), deviations of the lateral image position (uniform displacement of image points in the image field), image rotation, asymmetrical imaging scale, deformations of the focus position (non-uniform image point displacement perpendicular to the image plane), etc., but also variations of the critical dimensions over the image field (CD variations), differences in the critical dimensions in mutually orthogonal directions (HV aberrations), etc. In general, these aberrations are not uniform over the image field, but rather vary within the image field. Distortion and deformations of the focal plane can lead to overlay aberrations (e.g. overlay aberrations between different patterns (or mask structures) and focus aberrations. The lithographic aberrations are influenced by various properties of the projection exposure apparatus or of the projection exposure process, including the substrate, the radiation-sensitive layer on the substrate, the projection ray provided by the light source, the mask and the projection system.
As the structure sizes to be produced are becoming smaller and smaller, lithographic aberrations that are still acceptable in the case of larger structures can also become important.